I live nearby (Sathorn - suanplu area).
I did the Silom thing at Songkran around 3 or 4 times. I don't recommend it.
No fun at all unless you just like being squashed in a crowd, moving a 1/10 mile per hour :).
People who took the BTS just froze into place, couldn't move back or forth. Just staring down to the mass below.
Better off having fun in the less crowded streets and neighborhoods, which is real good fun and no hassle.
Up to you as they say.
Cheers